1. Field of the Invention
The present invention relates to an image display apparatus, and specifically, to an image display apparatus capable of improving contrast.
2. Description of the Related Art
Conventionally, image display apparatuses using organic EL (electroluminescence) devices, which have a function of generating light by emission due to recombination of holes and electrons injected in an emission layer, have been proposed.
For example, such an image display apparatus includes plural pixel circuits arranged in a matrix form, a data line drive circuit for supplying brightness signals, which will be described later, to the plural pixel circuits via plural data lines, and a scan line drive circuit for supplying scan signals to the pixel circuits via plural scan lines. The scan signals are signals for selecting pixel circuits to which brightness signals are supplied via the data lines.
Further, the pixel circuit (for one pixel) has a function of emitting light by current injection and includes a light emitting device as the above-described organic EL device, a driver device for controlling current flowing in the light emitting device, and two or three switching devices. These driver device and switching devices are thin-film transistors (TFTs). Accordingly, the conventional image display apparatus has three-TFT configuration having three thin-film transistors (one driver device+two switching devices) or four-TFT configuration having four thin-film transistors (one driver device+three switching devices), for one pixel circuit.
FIG. 15A shows a configuration of a main part of an image display apparatus (for one pixel) proposed in Dawson et al., “Design of an Improved Pixel for Polysilicon Active-Matrix Organic LED Display”, Society of Information Display 1998 Digest, 1998, p. 11-14 (hereinafter, referred to as “Dawson et al”). In the image display apparatus shown in FIG. 15A, a data line supply circuit 102 has a function of supplying a brightness signal via a data line 101. A scan line drive circuit 104 has a function of supplying a scan signal for selecting a pixel circuit for supplying a brightness potential via a scan line 103. A power supply circuit 105 has a function of supplying a high-level potential to one electrode of a capacitor 112 and an electrode of a switching device 108. A reset control circuit 114 supplies a reset potential to a switching device 109 via a reset line 115. A drive control circuit 116 supplies a control signal to a switching device 118 via a drive control line 117.
Further, in the image display apparatus, a light emitting device 107, the driver device 108, the switching device 109, the capacitor 112, the switching device 118, a capacitor 119, and a switching device 122 form a pixel circuit for one pixel. The light emitting device 107 has a mechanism of emitting light by current injection and consists of the above-described organic EL device. The switching device 108 has a function of controlling current flowing in the light emitting device 107.
The driver device 108 has a function of controlling the current flowing through the light emitting device 107 according to the potential difference equal to or more than the drive threshold value applied between a gate electrode corresponding to a first terminal a source electrode corresponding to a second terminal, and a function of keeping the current flow through the light emitting device 107 during application of the potential difference. The driver device 108 consists of a p-type thin-film transistor and controls the emission brightness of the light emitting device 107 according to the potential difference applied between the gate electrode and the source electrode.
FIG. 18A shows a configuration of a main part (for one pixel) of an image display apparatus having two-TFT configuration proposed in J. L. Sanford et al., Proc. of IDRC 03 p. 38. Further, FIG. 18B shows a time chart for explanation of the operation thereof. In the image display apparatus shown in FIG. 18A, a switching device T1, a driver device T2, a capacitor Cs, and a light emitting device OLED are connected as shown in the drawing to form two-TFT configuration (switching device T1 and driver device T2). The switching device T1 and driver device T2 are thin-film transistors.
However, in the image display apparatus as proposed in Dawson et al, there has been a problem of reduction in contrast because the light emitting device emits light in the reset step resetting the potential applied to the gate electrode of the driver device at the time of previous light emission.
Thus, in the image display apparatus as described by J. L. Sanford et al, there are cases where current flows through the light emitting device OLED in the reset step. That is, such an image display apparatus having two-TFT configuration is not applied to practical use.
Accordingly, there has been a problem that the conventional image display apparatus still adopts three-TFT configuration or four-TFT configuration for practical use and the improvement in definition is difficult.